Method for controlling the run-up of a conveyor belt and drive device for a conveyor belt

ABSTRACT

A method for controlling the run-up of a conveyor belt BY increasing a setpoint value of the belt&#39;s speed with an increasing acceleration in a first time interval, and with a decreasing acceleration in a second time interval, thereafter plotting a curve of the increasing acceleration against time which has a positive curvature in the second interval, a second time derivative of the acceleration in the first and the second time interval is at least approximately constant and at least approximately equal and opposite.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a national application for International Application No.PCT/DE01/00026 which was filed on Jan. 5, 2001 and which published inGerman on Jul. 26, 2001, which in turn claims priority from 100 02563.3, which was filed on Jan. 21, 2000.

FIELD OF THE INVENTION

The invention relates to a method for controlling the run-up of aconveyor belt and to a drive device for a conveyor belt.

BACKGROUND OF THE INVENTION

Conveyor belt systems that are long in length, for example in the rangeof a few kilometers, have to be run-up and braked particularly gently,in order to avoid belt oscillations and excessively high belt tensions.The rotational speed of the drive motors is therefore controlled by acontrol device known as a soft run-up controller, in order to enable agentle runup of the rotational speed. For example, “M. B. Singh, TheRole of Drive System Technology in Maximizing the Performance andEconomics of Long Belt Conveyors”, bulk solids handling, vol. 14, 1994,pp. 695-702 discloses running-up the rotational speed of the drive drumand therefore the belt speed u* with a linearly rising acceleration a*in a first time interval. In order to end the acceleration phase, in asecond time interval t2 the acceleration a* is reduced to zero. Thisreduction is likewise carried out linearly. In other words, the firsttime derivative r of the acceleration a* (referred to in specialistlanguage as the jerk) is constant and positive in a first time intervalt1, and likewise constant and negative in a second time interval t2. Thecurve of the rotational speed (or speed v* against time) is thereforeformed by quadratic functions with inverse curvature which follow eachother directly, that is to say in which the first time interval t1 witha rising acceleration a*, and the second time interval t2 with a fallingacceleration a* follow each other directly.

The time curve used in the known method of the setpoint value of theacceleration a*, of the setpoint value of the speed v*, and of the firsttime derivative r of the setpoint value of the acceleration a* (jerk) isplotted in a graph in FIG. 4.

FIG. 5 is a graph of the motor torque M which results from the knownmethod, and also the real head and tail speed, vk and vh, respectively,of a conveyor belt having a length of 5000 m, given a time duration ofthe first time interval and of the second time interval of 20 s, and amaximum standardized acceleration of 0.05 s−1, which occurs during arun-up operation. In this case, the standardized acceleration is to beunderstood as the ratio of the actual acceleration to the final speed ofthe belt. In the FIG. 5, it can be seen that significant fluctuationsoccur both in the motor torque M and in the conveyor belt itself In thecase of long conveying lengths, this can lead to disruptive operatingstates.

It is also known to drive the band drive with an acceleration-time curvewhich runs in accordance with a sinusoidal curve. In this method, theacceleration is likewise increased continuously up to a maximum value,and reduced continuously from there. Here the first time derivative ofthe acceleration at the maximum, i.e., the jerk, is zero. Belt andtorque fluctuations occur particularly at the end of the run-up in thismethod.

In both methods, the problem results that at the end of the run-upoperation, overswings or underswings in the motor torque occur, whichalso result in a greater belt tension. Furthermore, in the case of thesecond method, it is not possible to move to a different rotationalspeed during the run-up with the same curve characteristic, since thecurve characteristic is defined unambiguously by the run-up time, whichis equal to the sum of the time duration of the first time interval andof the time duration of the second time interval, and of the predefinedfinal speed.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for controllingthe run-up of a conveyor belt in which the aforementioned problems arelargely avoided. In the novel method for controlling the run-up of aconveyor belt, the setpoint value of the belt speed is increased with acontinuously increasing acceleration in a first time interval, and isincreased with a continuously decreasing acceleration in a second timeinterval, with the curve of the acceleration plotted against the timehaving a positive curvature in the second time interval. Since, as aresult of the positive, that is to say upwardly concave, curvature ofthe acceleration, the jerk decreases continuously in the second timeinterval, and belt oscillations at the end of the acceleration phase canlargely be eliminated by a run-up program configured in this way.

In a preferred embodiment of the method according to the presentinvention, the first time derivative of the acceleration is at leastapproximately equal to zero at the end of the second time interval. Thisensures that the conveyor belt moves particularly gently and softly toits final speed. More specifically, the second time derivative of theacceleration in the first and in the second time interval is in eachcase at least approximately constant, and equal and opposite. Thismeasure makes it possible to interrupt the run-up program at any timeand, during the run-up, it is possible to change the predefined finalrotational speed or final speed, without resulting in a differentcharacteristic of the acceleration-time curve.

The apparatus which facilitates the novel method is a drive device whichcontains a motor for driving a driving drum. A control device isassigned to the motor which permits the setpoint value of the belt speedto be controlled during the run-up of the conveyor belt in such a waythat said belt speed increases continuously with continuously increasingacceleration in a first time interval, and decreases with a continuouslydecreasing acceleration in a second time interval. This occurs in such away that the curve of the acceleration plotted against the time has apositive curvature.

BRIEF DESCRIPTION OF THE DRAWINGS

Further preferred embodiments of the drive device are disclosedhereinbelow in conjunction with the drawing, in which:

FIG. 1 illustrates a drive device according to the invention in a basisschematic form;

FIG. 2 illustrates a graph in which the setpoint value of the speed, theacceleration, and the first time derivative of the acceleration of theconveyor belt are in each case plotted against the time;

FIG. 3 illustrates a graph in which the true belt speed at the tail andat the head, the setpoint value of the belt speed, the acceleration, andthe motor torque are likewise plotted against the time; and

FIGS. 4 and 5 in each case illustrate graphs analogous to FIGS. 2 and 3using a method known from the prior art for controlling the run-up of aconveyor belt.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, a conveyor belt 2 is driven by two driving drums 4.Deflection drums 6 are used to tension or deflect the conveyor belt 2.The driving drums 4 are assigned a motor 8, generally an electric motor.It is not necessary for each driving drum 4 to be driven by a separatemotor 8, as shown in the illustrated exemplary embodiment.

As illustrated, the motors 8 are connected to a control device 10, whichcontrols the motor rotational speed n to the set point value n* inaccordance with a predefined run-up program. The setpoint value of themotor rotational speed n* is proportional, depending on the transmissionratio of the gearbox, to the setpoint value of the rotational speed ofthe driving rollers 4, which is in turn proportional to the setpointvalue of the belt speed v* of the conveyor belt 2. In practice, duringthe run-up or braking, the setpoint value of the belt speed v*,predefined by the setpoint value of the motor rotational speed n*, doesnot correspond with the real belt speed vh and vk at the tail (driveside) or head (deflection drum at the end of the belt). This is becausethe conveyor belt 2 is not an ideal rigid body but in reality anelastically deformable body.

In FIG. 2, the acceleration a*, i.e., the first time derivative of thesetpoint value of the speed v*, increases continuously with a constantconvex curvature during the run-up, which reaches its maximum value atthe end of a first time interval t1 (first jerk time). The accelerationa* plotted in FIG. 2 does not correspond to the real acceleration of theconveyor belt 2, but is only the differential of the predefined timecurve of the setpoint value of the belt speed v*. The first timederivative of the acceleration a*, the jerk r, is positive and isreproduced by means of a straight line with a negative slope, whichintersects the abscissa at the end of the first time interval t1. Thesecond time derivative of the acceleration a* is therefore constant andnegative, and the jerk r is equal to zero at the end of the first timeinterval t1.

In a second time interval t2, the acceleration operation is completed,the curve of the setpoint value of the acceleration a* having apositive, that is to say an upwardly concave curvature. The accelerationa* decreases continuously, and at the end of the second time interval t2(second jerk time) reaches the abscissa tangentially. Hence the firsttime derivative of the acceleration a* is equal to zero at the end ofthe second time interval t1. Likewise, the speed setpoint v* ends gentlyin the final setpoint value v*end, the final setpoint value v*end beingdriven particularly gently by the jerk r decreasing continuously down tozero. The second time derivative of the acceleration a* is constant andpositive in the second time interval. In the exemplary embodiment, thejerk r in the first time interval t1, and the jerk r in the second timeinterval t2 are in each case predefined by straight lines, with slopesthat are equal and opposite.

Provided between the first time interval t1 and the second time intervalt2 is a third time interval t3, in which the setpoint value of the beltspeed v* increases linearly with a constant acceleration. Depending onthe duration of the first jerk time t1 or of the third time interval t3,it is possible to move to any desired end speeds without changing thecharacteristic of the curve. This is illustrated in FIG. 2 by the curvesdrawn in dashed lines for a situation in which the first jerk time isended at the time ts. The jerk r then jumps (in the ideal case) from avalue rs to a value rs′. The second time derivative of the jerk r fort>ts is constant and, in terms of magnitude, is equal to the second timederivative of the jerk r for t<ts. The value rs′ has to be determined insuch a way that the time integral (hatched areas) over the jerk rvanishes. The reduction of the acceleration a*′ to zero is then carriedout with exactly the same curve as that which is also present in the endphase—the bracketed portion of the curve a*. Since the course of the endphase of the run-up according to the present invention has a constantcurve shape, it is possible to drive to any desired final speeds vend*′,and the run-up can be terminated at any time without changing the curvecharacteristic when moving towards the final speed.

In FIG. 3, by using the plotted curves for the motor torque M and theactual speed of a conveyor belt at the head vk and at the tail vh, itcan be seen that belt and torque fluctuations are virtually suppressed.The relationships represented in FIG. 3 reproduce the run-up operationfor a conveyor belt on a typical belt system with a total jerk time of20 seconds and a maximum standardized acceleration of 0.05 s−1.

We claim:
 1. A method for controlling the run-up of a conveyor beltcomprising increasing a setpoint value of the belt's speed with anincreasing acceleration in a first time interval, and with a decreasingacceleration in a second time interval, plotting a curve of theincreasing acceleration against time which has a positive curvature inthe second interval, a second time derivative of the acceleration in thefirst and the second time interval being at least approximately constantand at least approximately of like absolute value and of opposite sign.2. The method according to claim 1, wherein a first time derivative ofthe acceleration is at least approximately equal to zero at the end ofthe second time interval.
 3. The method according to claim 1, whereinthe second time derivative of the acceleration is positive in the secondtime interval.
 4. The method according to claim 1, wherein the secondtime derivative of the acceleration is negative in the first timeinterval.
 5. The method according to claim 1, further comprisingincreasing a rotational speed setpoint, between the first time intervaland the second time interval, with an at least approximately constantacceleration in a third time interval.
 6. An apparatus for a conveyorbelt, comprising a motor for driving a driving drum and a control devicefor controlling a setpoint value of the belt speed during run-up of theconveyor belt, such that acceleration increases continuously in a firsttime interval and decreases continuously and with a positive curvaturein a second time interval, a second time derivative of the accelerationin the first and in the second time interval being in each case at leastapproximately constant and at least approximately of like absolute valueand of opposite sign.
 7. The apparatus according to claim 6, wherein afirst time derivative of the acceleration is at least approximatelyequal to zero at the end of the second time interval.
 8. The apparatusaccording to claim 6, in which the second time derivative of theacceleration is positive in the second time interval.
 9. The apparatusaccording to claims 7 and 8, wherein the second time derivative of theacceleration is negative in the first time interval.